To each kernel \(k\)  is associated a Hilbert space (RKHS) consisting of real-valued functions on the phase space. The Maximum Mean Discrepancy (MMD) between two probability densities \(f\)  and \(g\)  is then defined by [1]}:
\(\gamma _k(f,g)=\max _{\phi }\left|\int f(X)\phi (X)dX-\int g(X)\phi (X)dX\right| \) 
